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New Census of Distant, Dusty Galaxies

A 3-D projection of almost 300 galaxies in the census in the same part of the sky. The third dimension shows how many billions of years back in time we are seeing each galaxy, determined by observations from the Keck Observatory. At top are images from the Hubble Space Telescope of five galaxies in the census. Credit: ESA-C. Carreau.

Caitlin Casey, a Hubble Fellow based at IfA, recently led an international group of astronomers who conducted a census of the brightest, but until now unseen, galaxies in the distant Universe. This work brings astronomers one step closer to understanding how galaxies form and evolve.

These galaxies glow so brightly at infrared wavelengths that they would outshine our own Milky Way by hundreds, maybe thousands, of times. They are forming stars so quickly that 100–500 new stars are born in each galaxy every year, so scientists call them “starbursts.”

Unfortunately, these galaxies are nearly invisible at the wavelengths our eyes, and most telescopes on Earth, can see because they contain huge amounts of dust, which absorbs visible starlight. “They were detectable directly in the infrared only from observations of the Herschel Space Observatory.  Herschel is an infrared space telescope sensitive to wavelengths not observable from within Earth’s atmosphere,” explained Casey.

Finding these galaxies at visible wavelengths required using the 10-meter Keck telescopes on Mauna Kea, the largest optical telescopes in the world. Over the course of several nights the group was able to detect and measure distances to nearly 800 of these galaxies.

“Detecting these bright infrared galaxies used to be difficult, and a handful was plenty; now with Herschel we are finding them by the thousands, enabling a census like this,” commented Göran Pilbratt, the Herschel Project Scientist at the European Space Agency, which launched Herschel.

“For the first time, we have been able to measure distances, star formation rates, and temperatures for a brand new set of 767 previously unidentified galaxies. The previous similar survey of distant infrared starbursts only covered 73 galaxies. This is a huge improvement,” said Scott Chapman, a co-author on the studies.

“While some of the galaxies are nearby, most are very distant; we even found galaxies that are so far that their light has taken 12 billion years to travel here, so we are seeing them when the Universe was only a ninth of its current age. Now that we have a pretty good idea of how important this type of galaxy is in forming huge numbers of stars in the Universe, the next step is to figure out why and how they formed,” said Casey.

The galaxies might shine so brightly in the infrared as a result of the collision of two spiral-type galaxies, similar to the Milky Way and Andromeda Galaxies. Or they could be so bright because they are in a particularly gas-rich region of space, where galaxies form stars quickly due to constant bombardment from gas and dust. Scientists have been debating these two formation methods for quite some time, but don’t yet have a clear answer.

“It’s hard to figure out how most galaxies formed based on information from only a small part of the Universe, just like it’s hard to guess how big an elephant is if you only get a glimpse of its tail,” Casey said.

“Now that we have an accurate census of starbursting galaxies across a huge time period in the Universe’s history, we can start to piece together how these galaxies grew and evolved.”

Two papers detailing these results have been published in the Astrophysical Journal.